1. Field of the Invention
The present invention relates to a controller for an injection molding machine, and more specifically to changeover of an injection velocity in an injection process and changeover from an injection-velocity control in the injection process to a resin-pressure control in a pressure-holding process.
2. Description of Related Art
With regard to an injection process in the molding operation of an injection molding machine, the method of controlling the injection process by changing injection velocity (forward velocity of a screw) according to the position of the screw that advances in the axial direction to inject resin into the mold or according to injection pressure (resin pressure) during injection and moreover according to the elapsed time from the start of injection is widely employed. As to changeover from the injection process (injection velocity control) to the pressure-holding process (pressure control), when a set changeover point from the velocity to pressure control (screw position, injection pressure, and the elapsed time from the start of injection) is reached, the injection velocity control is switched to the pressure control, to thereby implement the control of pressure, namely holding pressure control.
In the case of a controller of an injection molding machine using a processor, including an electric injection molding machine that drives a screw by means of a motor, a screw position, injection pressure (resin pressure) or elapsed time from the start of injection is detected in every prescribed sampling period. A determination is then made as to whether the detected screw position, injection pressure or elapsed time has reached the injection velocity changeover point or the changeover point from the injection velocity control to the holding pressure control. If the point has been reached, an injection velocity changeover or changeover from the injection velocity control to the pressure control is made. Therefore, the fact that the screw point, injection pressure, or elapsed time from the start of injection, which is set as a changeover point, has been reached is detected after a delay of sampling period time at the maximum. The determination that the set changeover point has been reached is made always after a delay except when the screw position, injection pressure or elapsed time detected in a certain sampling period coincides with the set changeover point by chance. In result, the injection velocity changeover and the changeover to the holding pressure control are delayed.
As a method of compensating this delay and changing the injection velocity at the set changeover point, the method of predicting whether a command position in the next sampling period exceeds the set injection velocity changeover point, and based on the prediction, controlling a motion command to be outputted next has been well known (see JP 8-323824A).
There is another well known invention in which in order to prevent resin from being applied with surge pressure due to the inertia of a screw drive mechanism or the like when a filling process (injection velocity control) is switched to the pressure-holding process (pressure control), once the injection screw reaches a changeover position from the filling process to the pressure-holding process, a position command to move the screw backward from the command position by a prescribed amount is outputted, to thereby suppress the generation of surge pressure (see JP 2002-28960A and JP 2005-35039A).
According to the method of detecting and controlling the screw position, injection pressure (resin pressure) or elapsed time from the start of injection which is the changeover point in every sampling period, as described above, by the time when the changeover point is detected, the changeover point has already been passed. The injection velocity changeover and the changeover to the holding pressure control are not performed, and the changeover is made after a delay.